This invention relates to a fluid displacement apparatus of the scroll type, such as a scroll type compressor.
Scroll type fluid displacement apparatus are well known in the prior art. For example, U.S. Pat. No. 801,182 discloses a scroll type fluid displacement apparatus including two scroll members, each having a circular end plate and a spiral or involute element. These scroll members are maintained angularly and radially offset so that both spiral elements interfit to make a plurality of line contacts between the spiral curved surfaces to thereby seal off and define at least one pair of fluid pockets. The relative orbital motion of the two scroll members shifts the line contacts along the spiral curved surfaces, and, therefore, the fluid pockets change in volume. The volume of the fluid pockets increases or decreases depending on the direction of the orbiting motion. Therefore, this scroll type fluid displacement apparatus is applicable to compress, expand or pump fluids.
The principle of operation of a typical scroll type compressor will be described with reference to FIGS. 1a-1d. FIGS. 1a-1d schematically illustrate the relative movement of interfitting spiral elements to compress the fluid and may be considered to be end views of a compressor wherein the end plates are removed and only the spiral elements are shown.
Two spiral elements 1 and 2 are angularly and radially offset and interfit with one another. As shown in FIG. 1a, orbiting spiral element 1 and fixed spiral element 2 make four line contacts as shown at four points A, B, C, D. A pair of fluid pockets 3a and 3b are defined between line contacts D-C and line contacts A-B, as shown by the dotted regions. Fluid pockets 3a and 3b are defined not only by the wall of spiral elements 1 and 2 but also by the end plates from which these spiral elements extend. When orbiting spiral element 1 is moved in relation to fixed spiral element 2 by, for example, a crank mechanism, so that the center O' of orbiting spiral element 1 revolves around the center O of fixed spiral element 2 with a radius of O--O', while rotation of the orbiting spiral element is prevented, the pair of fluid pockets 3a and 3b shift angularly and radially toward the center of the interfitting spiral elements with the volume of each fluid pocket 3a and 3b being gradually reduced, as shown in FIGS. 1a-1d. Therefore, the fluid in each pocket is compressed.
The pair of fluid pockets 3a and 3b are connected to one another while passing from the stage shown in FIG. 1c to that shown in 1d. As shown in FIG. 1a, both pockets 3a and 3b merge at center portion 5 and are completely connected to one another to form a single pocket. The volume of the connected single pocket is reduced by revolution of center O' about center O as shown in FIGS. 1b, 1c and 1d. During the course of revolution, outer spaces which open in the state shown in FIG. 1b change as shown in FIGS. 1c, 1d and 1a to form new sealed-off fluid pockets in which fluid is newly enclosed.
Accordingly, if circular end plates are disposed on, and sealed to, the axial facing ends of spiral elements 1 and 2, respectively, and if one of the end plates is provided with a discharge port 4 at the center thereof as shown, fluid is taken into the fluid pockets at the radial outer portion and is discharged from discharge port 4 after compression.
In a conventional scroll type compressor of the above type, if it is desired to increase the displacement or compression capacity, either the number of turns in the spiral elements must be increased or the axial length or "height" of the spiral elements must be increased. However, disadvantages result from an increase in the axial length of the spiral elements or an increase in the number of turns. For example, if the number of turns in the spirals element is increased, the diameter of the compressor also is increased.
Another disadvantage occurs by increasing the axial length of the entire spiral elements. Since the axial length of the spiral elements in conventional scroll type compressors is uniform and the end surface of the circular end plate is flat, the displacement volume of the fluid is proportional to the axial length of the spiral elements. The displacement volume of a scroll type compressor generally is defined by the outer fluid pocket space formed between the terminal outer end portion of the scroll member and the mid-way or central fluid space. The displacement of the outer fluid pockets formed by the spiral elements then is reduced due to a change of crank angle as shown in FIG. 2, which shows the volume change in one fluid pocket as a function of orbital motion. Line a in FIG. 2 illustrates the operation of a spiral element having an axial length H.sub.1 and line b illustrates the operation of a spiral element having an axial length H.sub.2, wherein H.sub.2 is smaller than H.sub.1. Thus, the volume of the fluid pockets formed by spiral elements having an axial length H.sub.2 (line b) is smaller than the volume formed by spiral elements having an axial length H.sub.1 (line a).
During operation of such a conventional scroll type compressor, when the volume of the fluid pockets is reduced as shown in FIG. 2, the gas or fluid pressure, which is proportional to the sectional area of the compressed chamber, increases. The greatest pressure occurs in the mid-way or central portion of the spiral elements. However, since the minimum volume of the central fluid pocket must be formed as small as possible in order to reduce the reexpansion volume, an increase in the axial length of the spiral elements in order to increase displacement or compression capacity has the disadvantage of increasing the reexpansion volume.